Optical Comb Offers Laser Reference

Kevin Robinson

The gold standard of time or frequency measurements at the heart of most atomic clocks is the microwave transition in cesium atoms. Now researchers have coupled the stable frequency reference of a cesium microwave clock to a mode-locked Ti:sapphire laser system, creating an optical comb that can determine the output of other lasers.

Building on the work of Ted W. Hänsch of the Max Planck Institute for Quantum Optics in Garching, Germany, researchers at the institute, Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J., and the Joint Institute for Laboratory Astrophysics (JILA), a collaboration of the University of Colorado and the National Institute of Standards and Technology, compared the output of the atomic clock with the pulse repetition rate of the Ti:sapphire laser.

A matter of Arithmetic

Passing the laser output through a nonlinear fiber more than doubles the frequency of the femtosecond pulse, explained John L. Hall, the principal investigator in the JILA group. This enabled the researchers to obtain short-wavelength frequencies both directly from the fiber and by frequency-doubling the fiber output in the red wavelengths. A heterodyne beat of these two classes of comb lines stabilized the comb's optical frequencies. The researchers thus could relate these stable frequencies to the pulse repetition rate and define the optical frequency as a high harmonic number multiplied by the repetition rate.

To measure other frequencies, the output from a laser under test is combined with the comb spectrum. After it passes a diffraction grating, this unknown laser beam and the comb lines near it reflect into a photodetector, and the beat frequency is counted. From there it is a matter of arithmetic to determine if the beat frequency should be added or subtracted from the spacing of the combs, Hall said.

The method, which the researchers described in the May 29, 2000, issue of Physical Review Letters, is easier and cheaper than its predecessor, a process that compares the test laser against an array of frequency-locked lasers. Hall said that it took scientists in his group using this method nearly 10 years to perform the first measurement of a 633-nm laser. "Now we can measure an arbitrary laser in an afternoon -- and have time for a lot of coffee."